The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, more particularly to pacemaker, defibrillator and/or cardiovertor devices which are able to deliver to the heart stimulation pulses of low energy for the treatment of cardiac rate disorders, and even more particularly to the so-called xe2x80x9cmultisitexe2x80x9d prostheses, in which electrodes are placed in a plurality of distinct respective sites in the tissue.
Multisite prosthesis typically comprise at least one ventricular site and one atrial site, and are known as xe2x80x9cdouble chamberxe2x80x9d (right atrial stimulation and right ventricular stimulation) or, more generally, xe2x80x9ctriple chamberxe2x80x9d (right atrial stimulation and double ventricular stimulation) or xe2x80x9cquadruple chamberxe2x80x9d (double atrial stimulation and double ventricular stimulation) prosthesis. Multisite devices also include a prosthesis type which provides stimulation at two distinct sites in the same cavity, for example, a double stimulation of the left ventricle.
In addition to the treatment of cardiac rate disorders, it has been proposed to treat by stimulation disorders of the myocardial contraction, which are observed among patients having a cardiac insufficiency. These disorders may be spontaneous or induced by a traditional stimulation. One will be able in particular to refer to the study of J. C. Daubert et al., Stimucoeur, 25, nxc2x03, pp. 170-176 which gives a report on this subject. Daubert et al. proposed to stimulate simultaneously and permanently the left and right ventricles, for the re-synchronization of both ventricles. One often can observe spectacular results for patients having a Class III-type cardiac insufficiency, whose condition was not significantly, if at all, improved by the traditional treatments.
In the following discussion, the case of a stimulation of the lower cardiac cavities, i.e., the case of a double ventricular stimulation, is discussed because this case is the one which is the most unfavorable for the cardiac function of the patient. The mechanism which will be described can, however, affect in the same way, the upper cardiac cavities, and the solutions suggested could be applied mutatis mutandis to a double atrial stimulation.
After delivery of a stimulation pulse, a depolarization wave is propagated in the volume of the myocardium around the stimulation point. A consequence of this stimulation is the creation, after the passage of the wave, of a refractory period (about 250 ms). During the refractory period, the cardiac cells are no longer excitable and thus will not respond to a stimulation. This refractory period is followed, before the cardiac cells have returned to the normal state, by a transient period during which the cardiac cells are hyper-excitable. Thus, any stimulation falling into this transient period, whether of a natural (spontaneous) or a stimulated (induced) origin, can trigger the excitation state of a cardiac cell and another depolarization wave due to the electric instability of the cardiac cells, and thus generate an undesirable phenomenon of tachycardia, i.e., an abnormally high cardiac rate.
Another phenomenon owing to the operation of the multisite device is that stimulation which is simultaneous or with a slight delay on two or more sites can lead to the creation of areas presenting aberrations of the electrical conduction in the region where the two depolarization propagating waves meet. This can lead to the appearance of a zone known as a xe2x80x9cblockxe2x80x9d, in which the propagation of the depolarization wave will be slowed down, and perhaps even stopped.
In addition, a double atrial stimulation can generate the following phenomenon: the first stimulation will lead, by normal conduction, to the subsequent depolarization of the ventricle; the second stimulation, whose propagation will be delayed in the zone of the block, also will reach the ventricle, but, because of the delay, will reach the ventricle during a point in time when the atrium is no longer in its refractory period, and thus will cause an undesirable contraction involving a disorder of the cardiac rate which can result in a tachycardia crisis.
It is, therefore, an object of the present invention to overcome these various phenomena known as xe2x80x9cinduced tachycardia,xe2x80x9d or xe2x80x9creentry tachycardiaxe2x80x9d (collectively designated as xe2x80x9cTRExe2x80x9d) and pacemaker mediated tachycardia (xe2x80x9cPMTxe2x80x9d) which can be caused by multisite type cardiac stimulation devices such as pacemakers.
To this end, the present invention broadly concerns a device of the multisite type, i.e., in which electrodes are to be placed in at least two cardiac sites of the same chamber type, such as at least two ventricular sites, one right and one left, or in at least two atrial sites, one right and one left, or in at least two sites of the same cavity, or combinations thereof. The electrodes are to be connected to a cardiac signal collection circuit to detect a depolarization potential, as well as to a stimulation circuit which delivers stimulation pulses to at least certain of the aforesaid sites.
According to one aspect of the invention, the device comprises means for monitoring the heart rate, means for detecting an induced tachycardia condition, operating in response to the monitoring means, to detect the presence of an induced tachycardia if the heart rate exceeds a predetermined threshold for a length of time greater than a predetermined threshold, and means for temporarily modifying the operation of the device in the event of a detected induced tachycardia condition.
In a preferred embodiment, the detection means detects the presence of an induced tachycardia only if the heart rate has a rate of increase which exceeds a predetermined minimal value.
Modifying the operation of the device can advantageously be implemented by reducing the time of a stimulation delay that may exist between the two cardiac sites, e.g., the two ventricular sites (right and left), and/or the two atrial sites (right and left), and/or the two stimulation sites of the same cavity.
In the alternative or in addition, the modifying the operation of the device may inhibit the stimulation of at least one of the two cardiac sites, e.g., one of the right and left ventricular sites, and/or one of the two right and left atrial sites, and/or one of the two stimulation sites of the same cavity.
Advantageously, one can foresee that modifying the operating mode to inhibit the delivery of a stimulation pulse is preferably employed only after having attempted to reduce the time of the stimulation delay, which reduction was followed by the case of a persistent tachycardia.
Preferably after activation of the means to modify the operation of the device, the modification is maintained activated for a predetermined length of time, and then deactivated.
Advantageously, one can foresee that after a deactivation of the operation modifying means, the normal operating mode of the device (i.e., normal or preselected mode of operation in the absence of an induced tachycardia condition) is reactivated (i.e., restored). Then, in the event of a new detection of induced tachycardia condition, the operation modifying means will again be employed to modify the operation of the device as described so as to inhibit a persistence of the induced tachycardia condition.